PET

Question 1

Describe the process of Positron Emission Tomography (PET scan)

Answer 1

Isotope is bound to a carrier and is injected into the patient. Carrier attaches to the target tissue and the radiation is measured

Question 2

Describe what happens in the Emission process

Answer 2

Proton rich isotope decays to a stable state by converting a proton to a neutron. A Positron is emitted which annihilates an electron, releasing two photons in opposite directions. These photons are detected.

Question 3

Give some examples of scans which are better with Radioisoptes with short half lifes.

Answer 3

11C - 20.4 mins - Receptor studies Prostate cancer
13N - 10 mins - Blood flow
15O - Oxygen extraction, Myocardial perfusion
82Rb 1.27 mins - Myocardial Perfusion

Question 4

Give an example of a scan which uses and isotope with a longer half life

Answer 4

F18 - 110mins - Tissue Metabolism, B-amyloid (alzheimers)

Question 5

What are the 5 main advantages to PET

Answer 5

High Sensitivity, Uniform High resolution, Superior Attenuation correction, Superior correction, Superior quantification, High clinical sensitivity

Question 6

what is the main advantage to having a relatively long half life

Answer 6

allows for off-site production

Question 7

What is 18F - FDG (Flurodeoxyglucose) commonly used for and how does its structure allow it to be affective.

Answer 7

Structurally similar to Glucose, 18F is substituted for a hydroxyl group. It is taken up into tissues with a high metabolic rate.

Question 8

Describe the process of uptake of FDG into cells

Answer 8

Glucose transporters facilitate FDG uptake. FDG is phosphorylated to FDG-6-Phosphate, not metabolised in glycotic pathway. It becomes trapped because tumor cells do not have enough glucose-6-phosphate to reverse phosphorylation.

Question 9

What is 18F-FDG a good marker of

Answer 9

Cell differentiation, Proliferative potential, Aggressiveness, Grade of malignancy

Question 10

Describe the method of PET detection

Answer 10

Detector is a ring which surrounds the patient. Scintillation material used to detect 522KeV photons. Block detectors used reduce detector dead time. No collimeters

Question 11

What are the 6 main features of a Scintillator material

Answer 11

Good stopping power for 511KeV photons
High Z-material
Low self-absorption factor
Refractive index close to glass for coupling with the PMT
Short decay time
Robust and easy to manufacture

Question 12

What are the 4 main events which can affect the image quality of PET

Answer 12

Random events
Scatter events
Attenuation
Resolution

Question 13

What is and causes a random event

Answer 13

Finite time window means that two photons from separate decays can be detected as a single coincident event.

Question 14

How do we minimize random events occurring

Answer 14

Store all events, apply formula and subtract. But it requirs a large memory and processing time.
Delayed coincidence time window. All coincidence events processed into a delayed time window are known to be unrelated and can be used for randoms estimation.

Question 15

How are coincidence time windows and detector related

Answer 15

Faster detector will use a shorter time window and detect fewer random events. But a short window with a slow detector will reduce sensitivity.

Question 16

How do scatter photons affect the image

Answer 16

They will be detected as real events but will have the wrong LoR and position therefore blurring the image. Reduction in resolution and contrast.

Question 17

How is scatter corrected

Answer 17

Ideally by having an energy acceptance of 511KeV but energy resolution detector is poor. Also 2-D imaging. 3D imaging can have a scatter factor of 25-50% so mathematical correction is required

Question 18

What are the advantages of 2D and 3d imaging

Answer 18

2D: Lower sensitivity, Less scatter, Simpler to reconstruct.
3D: Higher sensitivity, More randoms, Complex reconstruction

Question 19

Discuss what makes the resolution limit of PET

Answer 19

Proton has finite range it will travel before annihilating, the angle of anihilation is not exact 180. Fundamental limit is 1.5-2.5mm but technical limitations make it 5-8mm

Question 20

What are the advantages of 2D and 3d imaging

Answer 20

2D: Lower sensitivity, Less scatter, Simpler to reconstruct.
3D: Higher sensitivity, More randoms, Complex reconstruction

Question 21

What is the equation for the Standard uptake value for PET and what are they used for

Answer 21

SUV = (Decay corrected Activity (kBq) / Tissue Volume(ml)) / (Injected Dose (kBq) / Body Weight (g)

Used to compare scans and grade disease.

Question 22

What are the 5 clinical benefits of PET

Answer 22

Early diagnosis, Identification of distant metastases, Monituring theraputic efficiancy, Prediction of response to therapy, Treatment planning in radiotherapy

Question 23

What is the equation for the Standard uptake value for PET

Answer 23

SUV = (Decay corrected Activity (kBq) / Tissue Volume(ml)) / (Injected Dose (kBq) / Body Weight (g)

Question 24

What role does a CT scan play in PET imaging

Answer 24

used for accurate attenuation correction & localisation infused imaging

Question 25

By acquiring a time sequence of PET images it is possible to perform in vivo quantification of tracer kinetics. This can be used to monitor physiological processes such as

Answer 25

Blood flow - with radioactive water | Metabolism - FDG

Question 26

Tracer Kinetic models do what

Answer 26

Look at time-course of a tracer (Time activity curve) within regions of interest.

Question 27

What are the drawbacks of Tracer kinetic models

Answer 27

Requires complex acquisitions, time consuming, NOt feasible with routine images

Question 28

What are the three assumptions of tracer molecules

Answer 28

Do not alter the physiology of the body, Tracer is in a steady state with the molecule it is emulating, No isotpe affects

Question 29

Describe the process if cyclotron production of ET isotopes

Answer 29

Positrons are produced by proton rich nuclei. To do this we need to add protons to a nucleus. Small cyclotrons provide a source of positively charged protons or deuterons. Utilize a magnetic field to ben moving charges into a semi-circular spiral. An applie electrical field provides the acceleration

Question 30

What is the main clinical use of PET imaging

Answer 30

Typically Oncology (tumour) but also Cardiology & Neurology. Pet scans give a very precise location of a mass. Also used to help diagnose Alzheimers Disease

Question 31

What are the two theories of Alzheimers

Answer 31

Tau & Amyloid hypothesis

Question 32

Describe the tau hypothesis

Answer 32

Abnormal Tau protein tangles are considred the primary cause. Forms neurofibrillary tangles. Uses Tau-roux (Methylthionine) is a tau aggression inhibitor.

Question 33

Describe the Amyloid hypothesis

Answer 33

Amyloid proteins cause B-amyloid deposits and tau pathology downstream. Considered primary cause of clinical dementia. Interventions target the amyloid pathway.

Question 34

How does 18F-Florbetapair work

Answer 34

FDA approved to help with Alzheimers diagnosis. Positve scans display, a loss of grey and white matter contrast, White matter tracks hard to see running from frontal to parietal lobe. White matter tracks are sometimes lost in temporal/occipital area. Grey matter in medial parietal has increase uptake